This invention relates to ferroelectric liquid crystal compounds of the smectic type. More particularly, it relates to mixtures of smectic liquid crystal compounds which exhibit a ferroelectric, chiral smectic C phase and which find application in electrooptic display devices.
The utilization of the properties of a ferroelectric smectic phase to effect a switching phenomenon in an electrooptic display device has been known and is described by N. A. Clark and S. T. Lagerwall in App. Phys. Lett. 36, 899 (1980). Such devices operate with low electric power consumption while providing a more rapid switching than is realized in electrooptic display devices dependent upon the properties of liquid crystals of the nematic type. Display devices of the ferroelectric type rely upon two tilted configurations of smectic liquid crystal molecules to provide two states of equal energy. Switching from one state to the other is accomplished by moving a boundary between the two domains by applying an electric field across a pair of electrodes sandwiching a layer of the ferroelectric liquid crystal material.
Various liquid crystal compounds of the ferroelectric smectic type have been reported and examples of such compounds are set forth, for example, by G. Decobert and J. C. Dubois in Mol. Cryst. Liq. Cryst., 1984, 114, 237-247; by J. W. Goodby and T. M. Leslie, in Liquid Crystals and Ordered Fluids, Edited by A. C. Griffin and J. F. Johnson, Plenum Press, Vol. 4, pp. 1-32; and in European Patent Application 0110299 A2, published June 13, 1984. In the aforementioned European Patent Application 0110299 A2, it is indicated that the appearance of ferroelectricity in a molecular structure is dependent upon two conditions - the presence of an optically active group and the presence of an electric dipole in a direction approximately perpendicular to the major axis of the liquid crystal molecule, to induce spontaneous polarization. In general, it is well recognized that the suitability and operating efficiency of an electrooptic device will be dependent upon the chemical structure of a liquid crystal compound employed therein and that such properties will be influenced by such molecular factors as rigidity or stiffness, morphology, crystallinity and intermolecular forces.
In the formulation of liquid crystal-containing compositions for use in electrooptic devices, it has been common to formulate mixtures of two or more liquid crystal compounds to provide one or more properties superior to that attainable from a single liquid crystal compound. In the case of nematic liquid crystal compounds, mixtures of different classes of nematic liquid crystal compounds to provide compositions having wider nematic temperature ranges, have been described, for example, in U.S. Pat. No. 4,090,975 (issued May 23, 1978 to R. E. Aldrich, et al.). Similarly, in the aforementioned European Patent Application 0110299 A2, there are disclosed mixtures of ferroelectric smectic liquid crystal compounds to provide compositions showing a lowered melting point and a thermodynamically stable smectic C phase in a temperature range including room temperature.
In formulating mixtures of liquid crystal compounds for improved properties, an improvement in one property is oftentimes accompanied by an adverse affect on another desired property. For example, a lowering in smectic freezing point resulting from an admixture of smectic liquid crystal compounds may be accompanied by an undesired lowering of the isotropic transition temperature which tends to negate the advantage of the lowered freezing. In other instances, improvement (widening) of mesomorphic temperature range will be minimal. The properties achievable by a combination of liquid crystal compounds will typically be largely unpredictable.